1. Technical Field
The present invention relates to an exposure mask for use in a photolithographic step and also relates to a method for fabricating a thin-film transistor using the exposure mask.
2. Related Art
An example of an exposure mask (that is, a photomask) is a gradient mask (or a multiple-gradient mask) in which the transmittance of exposure light varies at three or more levels. The gradient mask is useful in forming a gradient resist film having a plurality of regions with different thicknesses. The use of the gradient mask is effective in reducing the number of, for example, photolithographic steps of processes for fabricating thin-film transistors.
Japanese Unexamined Patent Application Publication No. 8-250446 (hereinafter referred to as Patent Document 1) discloses an exposure mask (gray tone mask) which is an example of the gradient mask. This exposure mask includes a chromium layer having portions which have different thicknesses and therefore have different transmittances for exposure light. Japanese Unexamined Patent Application Publication Nos. 2002151523 (hereinafter referred to as Patent Document 2) and 2007-72452 (hereinafter referred to as Patent Document 3) each disclose an exposure mask that has a striped pattern (a slit pattern or a grating pattern) or a dotted pattern such that portions thereof have different transmittances for exposure light.
In the exposure mask disclosed in Patent Document 1, the chromium layer needs to be formed so as to have such portions which have different thicknesses and therefore have different transmittances for exposure light. Since it is difficult to form the chromium layer such that the portions thereof have desired thicknesses, there is a problem in that the exposure mask may have undesired transmittances. There is also a problem in that the cost of manufacturing the exposure mask is high because the chromium layer needs to be precisely formed so as to have desired thicknesses. Alternatively, if exposure light is applied to a resist layer through the exposure mask disclosed in Patent Document 2 or 3, a pattern corresponding to the striped or dotted pattern is formed on the resist layer, that is, the striped or dotted pattern is transferred to the resist layer. This causes a problem in that the resulting resist layer has surface irregularities. In particular, exposure light with a shorter wavelength is likely to cause the transfer of the striped or dotted pattern to the resist layer.